Equilateral shells, are those whose sides are alike, as in the shells of the genus Pecten. Plate [II]. fig. 2, and Plate [VII]. fig. 14. This is also exemplified in the Pectunclus.
Inequilateral valves are shells whose sides are unequal; and of different shapes, as in the Mactra, Donax, &c.
Summit is the most elevated point of that part of the shell in which the hinge is placed. Plate [I]. fig. 4, i k.
In naming this the summit we do not follow the axiom of Linnæus, but because we consider it more properly the summit of the shell than the opposite extremity.
Base is the reverse of the above, or that part of the shell immediately opposite the summit. Plate [I]. fig. 6 and 7, d d.
Sides, the right and left parts of the valves. Plate [I]. fig. 6, c.
Posterior slope is that part of the shell in which the ligament is situated. In viewing the posterior slope in front, the beaks of the shell retire from view. Plate [I]. fig. 4, i.
Anterior slope, that part of the shell opposite the posterior slope; in viewing it in front, the beaks point to the observer. Plate [I]. fig. 4, k.
Disk, the convex centre of a valve, or most prominent part of the valve, suppose it with its inside lying undermost. Plate [I]. fig. 4, o.
Inside, the concave part of a valve. Plate [IV]. fig. 6, m.